1. Field of the Invention
The present invention relates to electrical connectors and more particularly to high I/O density connectors such as connectors that are attachable to a circuit substrate by use of a solder ball contact surface.
2. Brief Description of Earlier Developments
The drive to reduce the size of electronic equipment, particularly personal portable devices, and to add additional functions to such equipment has resulted in an ongoing drive for miniaturization of all components, especially electrical connectors. Efforts to miniaturize electrical connectors have included reductions in the pitch between terminals in single or double row linear connectors, so that a relatively high number of I/O or other lines can be interconnected by connectors that fit within the tightly circumscribed areas on circuit substrates that are allotted for receiving connectors. The drive for miniaturization has also been accompanied by a shift in manufacturing preference to surface mount techniques (SMT) for mounting components on circuit substrates. The confluence of the increasing use of SMT and the required fine pitch of linear connectors has resulted in approaching the high volume, low cost limits of SMT for mounting connectors that employ presently available mounting designs. The limit is being reached because further reductions in pitch of the terminals greatly increase the risk of bridging adjacent solder pads or terminals during reflow of the solder paste. Array electrical connectors have been proposed to satisfy the need for increased I/O density. Such electrical connectors have a two dimensional array of terminal tails and can provide improved density. However, these connectors present certain difficulties with respect to attachment to the circuit substrate by SMT techniques because the surface mount tails of most, if not all, of the terminals must be attached beneath the connector body. As a result, the mounting techniques used must be highly reliable because of the difficulty in visually inspecting the solder connections and repairing them, if faulty.
Mounting techniques for other electronic components have addressed the reliability of solder connections in hard to inspect positions. For example, integrated circuit (IC) mounting to plastic or ceramic substrates have increasingly employed solder balls and other similar packages to provide such a reliable attachment. In a solder ball technique, spherical solder balls attached to the IC package are positioned on electrical contact pads of a circuit substrate to which a layer of solder paste has been applied, typically by use of a screen or mask. The unit is then heated to a temperature at which the solder paste and at least a portion of the solder ball melt and fuse to an underlying conductive pad formed on the circuit substrate. This heating process is commonly referred to as solder reflow. The IC is thereby connected to the substrate without need of external leads on the IC.
While the use of solder balls and similar systems in connecting ICs to a substrate has many advantages, a corresponding means for mounting an electrical connector or similar component on a circuit substrate has recently become desirable. The use of such techniques in mounting electrical connectors has lagged the use in mounting ICs because the use of solder ball technologies in mounting an electrical connector or similar component to a circuit substrate presents complexities not encountered with IC mounting. For example, ICs that have employed solder balls generally present a flat attachment surface. By contrast, connectors usually do not present a flat attachment surface but rather present a series of elongated conductors, commonly referred to as terminal tail ends. Attachment of a solder ball to the small end surface presented by the tip of a terminal tail end presents manufacturing difficulties not present in the attachment of solder balls to a flat surface.
In addition to the manufacturing difficulties, connectors are generally more susceptible to solder joint stress due to the effects of differential Coefficients of Thermal Expansion (CTE) between the connector and the circuit substrate. This susceptibility is primarily due to size and geometry differences between a connector and an IC. For example, IC mounting surfaces are generally on the order of 2.5 centimeters square. Connector mounting surfaces, on the other hand, generally have a narrow width (e.g., 0.5 centimeters or less)and a much longer length (e.g. 5.0 centimeters or more). Primarily because of the relatively long length of the connector, differences in CTE between a connector and a circuit substrate potentially have a much greater effect on the solder joints than the differences in CTE between an IC and a circuit substrate.
Connectors attached to circuit substrates via solder ball techniques are also more susceptible to joint stress than a conventional SMT attachment technique. For example, a conventional SMT attaches connector terminal tails to a circuit substrate horizontally, providing more attachment surface area for the solder joint. The additional surface area of the solder joint in the conventional SMT technique is stronger and, consequently, more tolerant of differences in CTE between the connector, terminal tails and circuit substrate. A solder ball connection, on the other hand, attaches a connector terminal tail vertically to the circuit substrate with the end of the terminal tail directly mated to the circuit substrate, reducing the amount of attachment surface area. As a result of the smaller attachment surface, differences in CTE are much more likely to stress the terminal tail to circuit substrate joint resulting in failure or quality problems.
Furthermore, in most circuit substrate applications, the electrical component mounting surfaces of the surface mount connections must meet strict coplanarity requirements. Thus, the use of solder balls to attach a connector to a circuit substrate imposes the requirement that the solder balls are coplanar in order to ensure a substantially flat mounting interface. So that, in the final application the balls will reflow and solder evenly to a planar circuit substrate. Any significant differences in solder coplanarity on a given mounting connection can cause poor soldering performance when the connector is reflowed onto a printed circuit board. Accordingly, users specify very tight coplanarity requirements to achieve high soldering reliability, on the order of 0.1 to 0.2 mm for example. By providing a connection using a solder ball technique, the coplanarity requirements can be met and sometimes exceeded. Unlike conventional SMT, the solder ball can absorb variations in terminal tail length by changing shape upon the application of heat to bridge the gaps between the terminal tail ends and the circuit substrate connections.
The present invention recognizes that there is a need for an improved electrical connector apparatus and accompanying electrical connector construction techniques that address the shortcomings of present electrical connectors.